I would have thought that it didn't matter if all of the gases passed the sensor or not, because it measures the instantaneous ratio of air to fuel, not quantity of one or the other (correct me if I'm wrong). Whether 200cfm or 150cfm of exhaust gas passes the sensor shouldn't make any difference, as the ratio of air to fuel over that volume would still be the same.
Have I totally missed something here, cos it's seems rather simple ?